A free electron laser (FEL) emits coherent light. A synchrotron emits incoherent light. Coherent light is produced in the FEL by wiggling the electron beam back and forth many times by a device called the wiggler, which contains a sinusoidal or helical magnetic field. In the process of oscillating back and forth, the electron radiates away some of its energy. Coherence is caused when the oscillations of the electrons are physically coherent, or in phase, with each other.
In the synchrotron, light is produced by bending an electron beam with one or a few magnetic fields, but the light does not have any feedback mechanism on itself, so it does not become coherent.
Part of the limitation in the output of a free electron laser or a synchrotron is caused by the fact that the radiating electrons do not all have the same longitudinal or transverse energy. In the process of going through a wiggler or around a synchrotron magnet, the electrons begin radiating light out of phase with each other, and eventually they get so out of phase that they can no longer effectively radiate any of their energy. This is caused by the fact that the electron beam in the FEL or synchrotron is not a purely monochromatic electron beam. The beam has variations in the average energy, including both a longitudinal and transverse energy, which causes the electromagnetic waves that are produced to interfere with each other, thereby reducing their intensity.
This invention involves compensating for this variation in longitudinal and transverse energy by taking the electrons which would normally arrive at the output end of the device earlier and giving them a longer path through the process, so that they then remain in phase with the electrons that are naturally taking a longer path through the device. The electrons therefore remain in phase for a longer period of time, allowing them to emit more of their energy and light, and permitting the device to become more efficient in the production of light.
This invention is a method and apparatus to compensate for the varying path lengths of the relativistic electrons in the FEL to boost the output of coherent light or in a synchrotron to boost the incoherent light. In the wiggler, the electrons that take the shortest path lengths are the ones in the center of the distribution, and the ones near the outside edge take a longer path length. This invention proposes the introduction of an optical beam, or electromagnetic wave, near the axis of the FEL or synchrotron electron beam to cause the electrons in the center of the beam to oscillate at a higher rate than those away from the center of the beam. The optical beam is made by a laser, typically a CO.sub.2 laser or a neodymium yag laser, with very intense fields and with the electric field vector pointing transversely.
The addition of the optical beam would cause the electrons in the center of the distribution to oscillate at a higher rate than those in the outer part of the distribution. The net effect would be that the electrons in the center now have a longer path length in going through the wiggler than they would have if the electromagnetic wave were absent. As a result of the added optical beam, the electrons tend to remain in optical phase for a longer period of time as they pass through the wiggler and therefore more of their energy can be extracted in going through the wiggler. This results in higher amounts of light being produced and therefore a brighter optical beam from the FEL or synchrotron.
Synchrotron light sources and free electron lasers are very popular for the production of electromagnetic radiation in the full wavelength range. They're used for both scientific and commercial purposes. One potential application of the FEL or synchrotron is in exposing photo resist masks in the production of semiconductors. Free electron lasers also offer great potential in chemical production as a result of their ability to produce high average power at tunable wavelengths. The FEL may be tuned to a wavelength that causes a particular chemical species to resonate and forces specific bonds to break allowing the FEL to be used to drive a chemical reaction in a certain direction that it would not normally take.
Free electron lasers also offer increased performance in cutting and ablating applications as a result of the higher average power of the FEL, the higher efficiency that makes the FEL a more cost effective source, and the ability to tune to specific shorter wavelengths which would enhance the absorption of light on the surface of the material that is being acted upon.
An FEL or synchrotron also holds promise in medical imaging such as imaging tumors. The ability to tune the device permits choosing a wavelength that the tumor containing tissue is transparent to, allowing a very clear picture of the tumor. In this manner a tumor may be viewed in a non-invasive way and can be repeated on a regular basis to monitor the progress of therapies.
One of the limitations to these devices is the fact that the quality of the electron beam has a major effect on their performance. Part of the limitation in the output of these devices is caused by the fact that the electrons that are radiating light energy do not all have the same longitudinal or transverse energy. In the process of going through the wiggler or around the synchrotron magnet, they begin radiating light out of phase with each other, and eventually they get so out of phase that they can no longer effectively radiate any of their energy.
The sensitivity of FEL gain to the electron beam energy spread and emittance is a major limitation especially when wavelengths in the DUV to soft X-ray region are considered. At such short wavelengths the beam emittance and/or energy spread becomes a limiting factor in the performance of most practical devices. Many designs have resorted to very long wigglers or very high peak currents in a MOPA configuration to achieve the required gain since mirrors have limited reflectivity in this region. Early proposals to improve the FEL acceptance for such situations worked with dispersed electrons and involved wiggler modifications to introduce a gradient in the wiggler resonant field. Recent work by A. M. Sessler, D. H. Whittum, and Li-Hua Yu, as reported on page 309, Volume 68 (1992), of Physics Review Letters involves modifications of the electron beam momentum distribution by means of a FODO channel and accelerator cavities operating on the TM.sub.210 mode to establish a correlation between energy and amplitude of transverse oscillations. These suggestions have shown the potential to reduce demands on the accelerator energy and on wiggler length with concomitant cost savings. This invention proposes a different approach to accomplish a similar goal, that is reduce the negative impact of transverse motion of electrons in a wiggler. Sessler et al's suggestion was to have the electrons on the outside of the distribution have higher energy so as to better maintain coherence whereas this invention involves slowing down the core electrons to achieve better coherence.
U.S. Pat. No. 4,748,629 provides for an FEL where time delay is used to phase lock the device to produce phase correlations between laser pulses.
U.S. Pat. No. 4,742,522 shows an FEL with precorrecting an output beam for phase aberrations by passing a low powered beam through the same path as a high powered beam and then phase-conjugating the beam and injecting it into the amplifier at the same time as an output pulse from the laser.
U.S. Pat. No. 4,845,718 shows the suppression of unwanted sidebands in FEL laser pulses by introducing a time dispersion of the sideband to cause a time lag between the main wavelength and the sideband.
None of the aforementioned patents teach the novelty of this invention, specifically the addition of an optical beam to change the path length of the core electrons of the FEL or synchrotron to boost their power output and efficiency.